The expression “interior surface” of a fluidized bed polymerization reactor system (or reactor) herein denotes a surface of the reactor system (or reactor) that is exposed to a reactant, recycle gas, and/or polymerization product during performance of a polymerization reaction in the reactor system (or reactor).
The expression “bed wall” is used herein to denote the portion or portions of the interior surfaces of a fluidized bed gas phase polymerization reactor system (or reactor) that is or are in contact with the fluidized bed during normal polymerization operation of the reactor system (or reactor). For example, typical embodiments of the invention pertain to treating the bed wall of a fluidized bed polymerization reactor preliminary to forming a polymer coating on the treated bed wall. The treatment applies a catalyst (in solution) to the bed wall so that the polymer coating can be formed by a special polymerization reaction in the presence of the applied catalyst. The special polymerization reaction is not the normal polymerization reaction to be performed in the reactor after the polymer coating has been formed.
The expression “solution catalyst” is used herein to denote a solution of at least one catalyst in at least one solvent. For example, chromocene (or another polymerization catalyst) dissolved in an aromatic solvent, such as, toluene (or another solvent) is a solution catalyst.
The term “comprises” is used herein to denote “is or includes.”
Gas phase polymerization of monomers, for example, olefin monomers, may be prone to forming “sheets” on the walls of the reactor vessel, particularly on certain catalyst types. Sheeting refers to the adherence of fused resin and resin particles to the walls or the dome of a reactor. The sheets vary widely in size. Sheets may be ¼ to 1/2  inch thick and may be from a few inches to several feet long. They may have a width of 3 inches to more than 18 inches. The sheets may have a core composed of fused polymer, which is oriented in the long direction of the sheets, and their surfaces are covered with granular resin that has fused to the core. The edges of the sheets often have a hairy appearance from strands of fused polymer. Sheeting rapidly plugs product discharge systems and/or disrupts fluidization, leading to the need for costly and time-consuming shutdowns.
Gas phase processes have been found to be particularly prone to sheeting when producing polymers using Ziegler-Natta catalysts, particularly Type III and Type IV Ziegler-Natta catalysts, certain bimodal catalyst systems, and catalyst systems containing metallocene catalyst compounds. While metallocene catalysts yield polymers with unique characteristics, they also present new challenges relative to traditional polymerization systems, in particular, the control of reactor sheeting.
A correlation exists between reactor sheeting and the presence of excess static charges, either positive or negative, in the reactor during polymerization (see, for example, U.S. Pat. Nos. 4,803,251 and 5,391,657). This is evidenced by sudden changes in static levels followed closely by deviation in temperature at the reactor wall. When the static charge levels on the catalyst and resin particles exceed critical levels, electrostatic forces drive the particles to the grounded metal walls of the reactor. The residency of these particles on the reactor wall facilitates melting due to elevated temperatures and particle fusion. Following this, disruption in fluidization patterns is generally evident, such as, for example, catalyst feed interruption, plugging of the product discharge system, and the occurrence of fused agglomerates (sheets) in the product.
It has been found that the presence of polymer coating on the bed wall of a gas phase (fluidized bed) polymerization reactor is desirable for reducing the tendency of the reactor to form sheets. Without being bound by theory, it is believed that the presence of certain reactor wall coatings (e.g., polymer coatings) inhibits the triboelectric charge transfer that would otherwise occur as the resin in the fluidized bed rubs against the metal reactor walls. Without being bound by theory, it is further believed that inhibiting the triboelectric of charge transfer has the effect of minimizing (or reducing) the accumulation of electrostatic charge on the resin. It is well known the accumulation of electrostatic charge on the resin can contribute to the formation of sheets in the reactor.
Fluidized bed polymerization reactors are often constructed of carbon steel, typically rated for operation at pressures up to about 30 bars (about 3.1 megapascals), and have interior surfaces composed of carbon steel. The normal appearance of the interior surfaces is that of plain, uncoated metal. However, a thin coating of polymer always (or almost always) forms on the bed wall of a fluidized bed polymerization reactor that has been in service. The coating is usually thin and relatively clear so that its presence is difficult to detect visually, but its presence can be detected with an Eddy current-type meter. The coating is normally composed of relatively low Mw (molecular weight) polymer and has a thickness of 1 to 20 mils (25 to 500 microns). Even though it is very thin, the coating has a significant effect on the operability of the reactor through its effect on the static charging characteristics of the fluid bed.
It is generally recognized that during fluid bed polymerization, fluid beds of polymer and other materials become charged by frictional contact with the reactor wall through a process known as the triboelectric effect. The charging mechanism depends on two factors: the nature of the materials involved, and the degree of contact. The basic driving force for transfer of charge is the difference in electrical characteristics of the two materials that contact each other. If there were no difference between the materials involved (e.g., if the two materials contacting each other were identical, for example, if both were carbon steel) no (or minimal) charge transfer would take place. In general, larger amounts of charge are transferred when the two materials in frictional contact are more different in their electrical characteristics (i.e., when they are far apart on the triboelectric series).
In gas phase polymerization reactors, the fluid bed can become highly charged through the frictional contact of two dissimilar materials, typically frictional contact between the polymer resin in the bed and the carbon steel of the bed wall. It is known that a good quality polymer coating on the bed wall acts to reduce the charging substantially, and thereby reduces the tendency for sheets to form on the bed wall. Some believe that the polymer coating is more similar in nature to the polymer in the fluid bed (compared to the carbon steel), thus reducing the driving force for charge transfer in the triboelectric process. Whatever the reason, it is clear that the coating on the bed wall (and possibly also other interior surfaces of the reactor system) has a significant effect on the static charging characteristics of the fluid bed.
When the polymer coating on the bed wall is in “good” condition, as indicated by its charge decay characteristics, a fluidized bed reactor system can be operated for extended periods of time (months or years) without excessive static and without operational problems due to sheeting. A reactor in this state is said to have a good static baseline, is relatively insensitive to the type of product being produced (e.g., its molecular weight “Mw” and density), and can typically be operated to produce the full range of polyethylene (PE) resin grades without generating excessive levels of static charge or sheeting.
However, when the bed wall coating is in “poor” condition, a considerable amount of static activity can develop in the fluid bed, which often leads to sheeting. A reactor in this state is said to be “sensitive” because the static charging characteristics become highly sensitive to the Mw and density of the product being produced.
The factors that cause the polymer coating on a bed wall to change from good to bad have been investigated from several different aspects. For example, it is known that the coating can deteriorate during normal polymerization operation and maintenance by exposure to aluminum alkyls compounds followed by repeated or prolonged exposure to water and air when the reactor is opened for maintenance. Aluminum alkyl compounds that are known to cause deterioration include methyl and ethyl alumoxane, triethyl aluminum and trimethyl aluminum. The alumoxanes are commonly used in metallocene polymerization and include bound trimethyl aluminum. Trimethyl and triethyl aluminum are commonly employed as cocatalyst in Ziegler-Natta polymerization. Water reactions with organoaluminum are the origin of the deterioration. It has been experimentally confirmed that μ-oxo compounds are formed which quickly deactivate to form a particular hydrated species of alumina called boehmite and represented chemically by Al(O)OH.
It is also suspected that prolonged exposure to impurities can lead to wall film degradation. These impurities include C6 oxides such as hexanol, and 1,2 hexanediol, both of which are reaction products of 1-hexene and oxygen. Thus, it is hypothesized that the deterioration in bed wall coatings may involve an oxidation of the polymer coating.
Although, in most cases, it is not certain what is the exact mechanism or mechanisms that cause the deterioration, it is well known a polymer coating on the bed wall can be deteriorated or contaminated over time, and this can have a major effect on operability of the reactor.
In practice, the reactor static baseline does not change suddenly. Rather, coating contamination or deterioration usually occurs over a period of time. As this happens, static activity and sheeting problems gradually develop and appear first during the production of certain resin products. These products, usually characterized as having higher molecular weights and higher densities, are referred to as the sensitive reactor grades. With a relatively mild degree of reactor wall contamination, static and sheeting problems are initially seen with the highest Mw products and some of the higher density grades. As the static baseline deteriorates further (e.g., as the wall coating becomes more contaminated) static and sheeting problems begin to occur with more and more products. The sensitivity of sheeting risk to different resin grades appears only with a contaminated or deteriorated bed wall coating. If the coating is in good condition, static remains near zero for all products.
Two types of reactor system retreatments, for removal of a bad (deteriorated or contaminated) bed wall coating and replacement with a new polymer coating, have been used commercially. Both retreatment methods involve preparation of the bed wall (typically by removal of an existing bad polymer coating) and the in situ creation of a new polymer coating on the wall. These conventional techniques have proven effective to some degree and with some catalyst systems.
One type of conventional retreatment method is known as chromocene treatment. To perform such retreatment, the bad (e.g., contaminated) polymer coating is removed from the bed wall by grit blasting. The reactor is then sealed and purged with nitrogen to remove oxygen and moisture. A solution catalyst (chromocene in solution) is then introduced into the reactor and the catalyst deposits on the reactor wall. The catalyst on reactor wall is then activated by controlled oxidation, purging, and then introducing ethylene and an alkyl such as tri-ethyl aluminum to form a new polymer resin coating (preferably a high molecular weight polymer coating) on the bed wall that may be effective in reducing charge buildup on the reactor bed wall and impeding sheet formation. The solution catalyst may include any of various chromium compounds (e.g., bis-cyclopentadienyl chromium and other chromocenes). U.S. Pat. Nos. 4,532,311, 4,792,592, and 4,876,320, for example, disclose methods of reducing sheeting in a fluidized bed reactor by introducing a chromium-containing compound into the reactor prior to a special polymerization reaction (catalyzed by the chromium) to form the high molecular weight coating on the bed wall of the reactor.
Another type of conventional reactor retreatment (for restoring a previously formed polymer coating) is known as hydroblasting. In this method, a contaminated or damaged polymer coating is removed from the bed wall with a high-pressure water jet. The reactor is then dried and purged with nitrogen and restarted in the normal fashion, but with a relatively high concentration of hydrogen so as to produce (by polymerization) a high melt index material (the melt index or “MI” is typically 10 or more as measured by the I2 method). The high melt index, low Mw resin produced readily deposits on the reactor bed wall, producing a new polymer coating which reduces the risk of sheeting during subsequent normal polymerization operation of the reactor.
We next describe typical conventional chromocene retreatment methods in more detail. After the bed wall is cleaned (e.g., by grit blasting) and the reactor is sealed and purged, such methods include the step of injecting a chromium-containing compound in solution (e.g., chromocene dissolved in toluene) into the reactor and circulating the injected compound so that some of the catalyst is deposited on the reactor's bed wall. The deposited catalyst is then oxidized, and the reactor is then opened for cleaning. The next step in this retreatment method is to purge the reactor with nitrogen and then activate the deposited catalyst by introducing ethylene and an alkyl to the reactor. The chromium-containing compound (e.g., chromocene) acts as a catalyst to polymerize the ethylene in the presence of alkyl to form the coating.
In conventional chromocene treatment methods, it is desired that the chromocene-containing solution (e.g., chromocene dissolved in toluene) will contact the reactor's bed wall to deposit the chromocene on the bed wall. It is generally believed that the concentration of chromocene in the solvent is not critical to the process, and this concentration is typically selected to assure that the chromocene is completely dissolved in the solvent. A solution containing about 5 to 8 percent by weight of chromocene in toluene is commonly used.
Referring to FIG. 1, conventional deposition of chromocene on the interior surfaces of a gas phase polymerization reactor 4 is typically done by injecting a chromocene containing solution through a feed tube at each one of a set of catalyst injection points 2. One such feed tube is shown at point 2 in FIG. 1. At each injection point, the solution may be injected through a single straight tube or through a tube with a spray nozzle at its end. An inert gas, such as nitrogen, is circulated through reactor 4 by cycle compressor 6 while the solution is slowly injected over a period of time (typically at least one to three hours, and sometimes as long as eight hours). The reactor system then circulates the mixture for a relatively long time (e.g., about twenty hours). It has been found that the level of chromium deposited on the bed wall by such a conventional method is typically significantly lower than the level of chromium deposited in the bottom head and on the bottom of the reactor's distributor plate 10. The method preferentially deposits the chromium on distributor plate 10 and in various parts of the reactor system other than the bed wall, such as in cycle compressor 6 and cycle cooler 12. The chromium deposited on distributor plate 10 (and other parts of the reactor system other than the bed wall) by the prior art method typically must be cleaned off before reacting the chromium to form the desired polymer coating.
The polymer coating formed on the bed wall of a fluidized bed polymerization reactor after chromocene treatment is intended to function as an insulating layer that reduces static charging in the reactor system, thereby reducing the potential for sheeting during subsequent normal polymerization reactions. Although typically thin (e.g., about 1 to about 20 mils, or 0.025 to 0.50 millimeters, where one “mil” denotes 0.001 inches), such a polymer coating can be effective in reducing static charging and is typically also durable. Often, a typically thin polymer coating of this type has a service life of at least four years before another retreatment is required, if (as is typical) the coating consists of a high density, high molecular weight (very low melt index) polymer. Such a coating having high density, high molecular weight, and low melt index, is typically highly resistant to abrasion by the softer polymer typically present in the fluid bed during normal polymerization operation.
The polymer coating formed on the bed wall of a fluidized bed polymerization reactor by conventional chromocene retreatment typically does not have uniform thickness throughout the bed wall. Without being bound by theory, the inventors believe that the conventional methods do not provide a uniform polymer coating on the bed wall because the chromium containing compound is not deposited uniformly on the bed wall.
Although conventional chromocene retreatment methods can form effective and reliable polymer coatings on the bed walls of fluidized bed polymerization reactors, they do not reliably form such effective and reliable coatings. Often, such conventional methods fail to form effective and reliable polymer coatings and instead form little or no polymer on a bed wall (or on portions of a bed wall). Without an effective polymer coating, a reactor that has undergone such failed treatment is sensitive to static charging and sheeting, particularly during polymerization reactions using metallocene catalysts.
The inventors have recognized that conventional application of chromocene solution (or other solution catalyst) during conventional retreatment methods allows the solution catalyst to evaporate (or undergo sublimation) before contacting the bed wall, so that the catalyst is not applied to the bed wall in the form of liquid droplets. This prevents the conventional methods from reliably forming effective, reliable polymer coatings on the bed wall.
What is needed is a more reliable method for forming effective and reliable polymer coatings on the bed walls and other interior surfaces of fluidized bed polymerization reactors.
Fouling problems often result from the performance of methods that include steps of applying solution catalyst to interior surfaces of a polymerization reactor system and then performing a polymerization reaction (catalyzed by the applied catalyst) to form a polymer coating on each surface. Specifically, excessive amounts of the polymer coating material can foul components of the system. Some reactor system components (e.g., distributor plates and compressor bases) are particularly vulnerable to this type of fouling. It would be desirable if such methods could be modified to reduce or eliminate such fouling of reactor system components with polymer coating material.